The invention relates to an apparatus and method of sensing the perforations in a perforated strip, in order to generate pulses used for various control functions, such as controlled transport by predetermined distances, automatic regulation of strip transport speed, and many others.
Known perforation sensing arrangements are not readily capable, if at all, of distinguishing between forward and backward transport of a perforated strip, i.e., merely from the sensing pulses generated in response to detection of a perforation in a transported strip.
Prior-art arrangements capable of differentiating between forward and backward strip transport incorporate, for example, a moving signal generator driven by the strip transport means and two photoelectric transducers arranged one after the other in the strip transport direction in the region of a sensing beam. The transducers supply pulses offset from each other with respect to time and partly overlapping. Depending upon which of the transducers first receives light, an evaluating circuit generates one or the other of two different pulses, utilized for example to effect forward or backward counting on a counter.
Despite the considerable expense involved for the moving signal generator, sensing arrangements of this type are not very satisfactory. The strip transport means which also drives the moving signal generator will often be running idle -- i.e., when a strip is not being transported. As a result, even when the strip transport means is running idle, the signal generator continues to generate pulses. Special means are required to prevent generation of this misinformation or to discriminate between this misinformation and proper information, for example means operative for automatically turning the signal generator off when the strip transport means ceases to transport a strip and begins to run idle, and for then automatically turning the signal generator back on when the strip transport means resumes strip transport. Additionally, with this known type of sensing arrangement, no means are provided for taking into account discrepancies between the speed of operation of the strip transport means and the true strip transport speed. The resultant misinformation cannot be corrected or compensated for.